Is it I9000 or I9000T? If it's I9000 no amount of modem firmware changing will get it to work on Telecom. The two models have physically different radios and only the I9000T is compatible with the 850mhz band for use on Telecom.

The galaxy s came in 2 different models either 850mh 3g for Telecom or 900mhz 3g for voda/2deg.
All 3 companies also use 2100mhz 3g in some areas for extra bandwidth which both galaxy s models also support.
The galaxy s was pretty much the last top-end Android phone that didn't support both 850 & 900 MHz 3g in the same radio chip. Nowadays most phones have both, but always check on gsmarena.com to be sure.

In my personal opinion the checker shouldn't say if you're phone has 2100 and thus "may" work on the network. If your phone doesn't support WCDMA/UMTS 850Mhz (GSM 850Mhz doesn't count) then it's not compatible on XT. Just because it works in the Metro areas and you can find a signal doesn't mean that your coverage will be at all reliable which means you blame the network for having poor coverage when your phone technically isn't supported on the network.

I'm sorry to say that if it doesn't have the T in the model number, then it's not compatible due to the radio not supporting WCDMA/UMTS 850Mhz.

To confirm your device will work on the XT Mobile Network, please visit a Telecom store or dealer to try your device with a Telecom SIM.

Now i'm just confused. The sticker definitely says i9000 on the back and not i9000t and yet the website seems to indicate that based on the IMEI it should work.

Hummm sounds wrong to me. If you want to email me your IMEI and some more details to "pl at telecom dot co dot nz" and I can look further into it. But it does sound like you have the non WCDMA 850 phone :(.

The Galaxy S i9000 is not compatible with Telecom NZ as the frequency on the phone run's on 900/2100 . Telecom NZ network run's on an 850/2100 MHZ. They are a 3G network Only, so there is no 2G network.

The Galaxy S i9000T has the 850/2100 MHZ frequency, Telstra AU and Telecom NZ have this variant as it is the main requirement of their network.

Basically your Phone will not work well on telecom's network, coverage will be limited to the main city centres (2100 MHZ) .

I would not recommened using that tool , as it can be quite confusing at times.

This also goes for alot of the phone's aswell. Like the HTC Sensation, HTC Wildfire S , Samsung Galaxy Series , etc. Alot of these phone's usually only come in the 900/2100 variant. So if you have not bought it from telecom nz, chances are it wont work well.

Bare in mind , most high end android devices come in quad band and penta band 3G / UMTS frequencys.